Counting
the days - tiny routines for converting Gregorian dates
into linear counts, like Julian day numbers or Unix time_t.
The date of the
count - a small routine for converting linear day counts
into Gregorian dates.

Exim is
Turing-equivalent.
I wrote a little demo
which includes an informal description of how to translate a
Turing machine into an Exim configuration, and an example
configuration that implements combinator reduction like my IOCCC
winner mentioned above.